Introduction to Database Systems

Introduction to Database Systems

The slides for this text are organized into chapters. This lecture covers Chapter 1.

Chapter 1: Introduction to Database SystemsChapter 2: The Entity-Relationship ModelChapter 3: The Relational ModelChapter 4 (Part A): Relational AlgebraChapter 4 (Part B): Relational CalculusChapter 5: SQL: Queries, Programming, TriggersChapter 6: Query-by-Example (QBE)Chapter 7: Storing Data: Disks and FilesChapter 8: File Organizations and IndexingChapter 9: Tree-Structured IndexingChapter 10: Hash-Based IndexingChapter 11: External SortingChapter 12 (Part A): Evaluation of Relational OperatorsChapter 12 (Part B): Evaluation of Relational Operators: Other TechniquesChapter 13: Introduction to Query OptimizationChapter 14: A Typical Relational OptimizerChapter 15: Schema Refinement and Normal FormsChapter 16 (Part A): Physical Database DesignChapter 16 (Part B): Database TuningChapter 17: SecurityChapter 18: Transaction Management OverviewChapter 19: Concurrency ControlChapter 20: Crash RecoveryChapter 21: Parallel and Distributed DatabasesChapter 22: Internet DatabasesChapter 23: Decision SupportChapter 24: Data MiningChapter 25: Object-Database SystemsChapter 26: Spatial Data ManagementChapter 27: Deductive DatabasesChapter 28: Additional Topics

What Is a DBMS?A very large, integrated collection of data.Models real-world enterprise. Entities (e.g., students, courses) Relationships (e.g., Madonna is taking CS564)A Database Management System (DBMS) is a software package designed to store and manage databases.

Historical PerspectiveEarly 1960sIntegrated data store, first general-purpose DBMS designed by Charles Bachman at GEFormed basis for network data modelBachman received Turing Award in 1973 for his work in database area

Historical PerspectiveLate 1960sIBM developed Information Management System (IMS), used even today in many major installationsIMS formed the basis for hierarchical data modelAmerican Airlines and IBM jointly developed SABRE for making airline reservationsSABRE is used today to populate Web-based travel services such as Travelocity

Historical Perspective1970Edgar Codd, at IBMs San Jose Research Laboratory, proposed relational data model.It sparked the rapid development of several DBMSs based on relational model, along with a rich body of theoretical results that placed the field on a firm foundation.Codd won 1981 Turing Award.Database systems matured as an academic disciplineThe benefits of DBMS were widely recognized, and the use of DBMSs for managing corporate data became standard practice.

Historical Perspective1980sRelational data model consolidated its position as dominant DBMS paradigm, and database systems continued to gain widespread useSQL query language, developed as part of IBMs System R project, is now the standard query languageSQL was standardized in late 1980s, and current standard SQL:1999 was adopted by ANSI and ISO

Historical PerspectiveLate 1980s till 1990sConsiderable research into more powerful query language and richer data model, with emphasis on supporting complex analysis of data from all parts of an enterpriseSeveral vendors, e.g., IBMs DB2, Oracle 8, Informix UDS, extended their systems with the ability to store new data types such as images and text, and to ask more complex queriesData warehouses have been developed by many vendors to consolidate data from several databases, and for carrying out specialized analysis

File Systems vs DBMSMust write special programs to answer each question a user may want to ask about dataMust protect data from inconsistent changes made by different users accessing data concurrentlyMust cope with system crashes to ensure data consistencyNeed to enforce security policies in which different users have permission to access different subsets of the data

Why Use a DBMS?Data independence (see next page) and efficient access.Reduced application development time.Data integrity and security.Uniform data administration.Concurrent access, recovery from crashes.

3

Program-data dependence --- Three file processing systems at Some Company

File descriptions are stored within each application program thataccesses a given file. Any change to a file structure requires changesto the file descriptions for all programs that access the file.

Why Study Databases??Shift from computation to informationat the low end: scramble to webspace (a mess!)at the high end: scientific applicationsDatasets increasing in diversity and volume. Digital libraries, interactive video, Human Genome project, EOS project ... need for DBMS explodingDBMS encompasses most of CSOS, languages, theory, AI, multimedia, logic

?

Data ModelsA data model is a collection of concepts for describing data.A schema is a description of a particular collection of data, using the a given data model.The relational model of data is the most widely used model today.Main concept: relation, basically a table with rows and columns.Every relation has a schema, which describes the columns, or fields.

5

Levels of AbstractionMany views, single conceptual (logical) schema and physical schema.Views describe how users see the data. Conceptual schema defines logical structurePhysical schema describes the files and indexes used.

Schemas are defined using DDL; data is modified/queried using DML.

Physical SchemaConceptual SchemaView 1View 2View 3

6

Example: University DatabaseConceptual schema: Students(sid: string, name: string, login: string, age: integer, gpa:real) Courses(cid: string, cname:string, credits:integer) Enrolled(sid:string, cid:string, grade:string)Physical schema:Relations stored as unordered files. Index on first column of Students.External Schema (View): Course_info(cid:string,enrollment:integer)

7

Data IndependenceApplications insulated from how data is structured and stored.Logical data independence: Protection from changes in logical structure of data (the capacity to change the conceptual schema without having to change external schemas or application programs). One of the most important benefits of using a DBMS!

Data Independence (cont.)Physical data independence: Protection from changes in physical structure of data (the capacity to change the internal schema without having to change the conceptual (or external) schemas). One of the most important benefits of using a DBMS!

Concurrency ControlConcurrent execution of user programs is essential for good DBMS performance.Because disk accesses are frequent, and relatively slow, it is important to keep the cpu humming by working on several user programs concurrently.Interleaving actions of different user programs can lead to inconsistency: e.g., check is cleared while account balance is being computed.DBMS ensures such problems dont arise: users can pretend they are using a single-user system.

Transaction: An Execution of a DB ProgramKey concept is transaction, which is an atomic sequence of database actions (reads/writes).Each transaction, executed completely, must leave the DB in a consistent state if DB is consistent when the transaction begins.Users can specify some simple integrity constraints on the data, and the DBMS will enforce these constraints.Beyond this, the DBMS does not really understand the semantics of the data. (e.g., it does not understand how the interest on a bank account is computed).Thus, ensuring that a transaction (run alone) preserves consistency is ultimately the users responsibility!

Scheduling Concurrent TransactionsDBMS ensures that execution of {T1, ... , Tn} is equivalent to some serial execution T1 ... Tn.Before reading/writing an object, a transaction requests a lock on the object, and waits till the DBMS gives it the lock. All locks are released at the end of the transaction. (Strict 2PL locking protocol.)Idea: If an action of Ti (say, writing X) affects Tj (which perhaps reads X), one of them, say Ti, will obtain the lock on X first and Tj is forced to wait until Ti completes; this effectively orders the transactions.What if Tj already has a lock on Y and Ti later requests a lock on Y? (Deadlock!) Ti or Tj is aborted and restarted!

Ensuring AtomicityDBMS ensures atomicity (all-or-nothing property) even if system crashes in the middle of a Xact.Idea: Keep a log (history) of all actions carried out by the DBMS while executing a set of Xacts:Before a change is made to the database, the corresponding log entry is forced to a safe location. (WAL protocol; OS support for this is often inadequate.)After a crash, the effects of partially executed transactions are undone using the log. (Thanks to WAL, if log entry wasnt saved before the crash, corresponding change was not applied to database!)

The LogThe following actions are recorded in the log:Ti writes an object: the old value and the new value.Log record must go to disk before the changed page!Ti commits/aborts: a log record indicating this action.Log records chained together by Xact id, so its easy to undo a specific Xact (e.g., to resolve a deadlock).Log is often duplexed and archived on stable storage.All log related activities (and in fact, all CC related activities such as lock/unlock, dealing with deadlocks etc.) are handled transparently by the DBMS.

20

Overview of System Architecture

Database CacheLog BufferStable DatabaseStable Log

DatabasePage

DatabasePage

Log Entry

Log Entry

readwritebegincommit, rollback

writefetchflushforceVolatileMemoryStableStorageDatabase Server

Databases make these folks happy ...End users and DBMS vendorsDB application programmersE.g. smart webmastersDatabase administrator (DBA)Designs logical /physical schemasHandles security and authorizationData availability, crash recovery Database tuning as needs evolveMust understand how a DBMS works!

21

Structure of a DBMSA typical DBMS has a layered architecture.The figure does not show the concurrency control and recovery components.This is one of several possible architectures; each system has its own variations.

Query Optimizationand ExecutionRelational OperatorsFiles and Access MethodsBuffer ManagementDisk Space Management

DB

These layersmust considerconcurrencycontrol andrecovery

22

SummaryDBMS used to maintain, query large datasets.Benefits include recovery from system crashes, concurrent access, quick application development, data integrity and security.Levels of abstraction give data independence.A DBMS typically has a layered architecture.DBAs hold responsible jobs and are well-paid!DBMS R&D is one of the broadest, most exciting areas in CS.